The present invention is directed to viscometers.
U.S. Pat. No. 4,627,272 to Wright describes a viscometer in which a bob containing ferromagnetic material is disposed inside an elongated chamber that contains the fluid whose viscosity is to be measured. A pair of coils is disposed around the chamber and spaced longitudinally of it so as to enable the coils to draw the bob alternately in opposite directions. The Wright viscometer determines viscosity by measuring the time required to drive the bob through the fluid from one end of the path to the other.
As is stated in that patent, it is preferable that the bob be of substantially neutral buoyancy. The advantage of neutral buoyancy is that it makes the viscometer insensitive to vibration and to the orientation of the viscometer with respect to gravity.
But exact neutral buoyancy cannot always be achieved. The density of the fluid whose viscosity is to be measured sometimes varies. Moreover, the design constraints imposed by certain applications may make it more desirable that the mass of the bob be very low, and the result may be that the bob has significant positive buoyancy. For these reasons, the orientation of the viscometer can affect the accuracy of its measurement. Furthermore, the viscometer may be placed in a flowing fluid, and the orientation of the viscometer with respect to the flow direction can affect the measurement in those arrangements in which the bob is exposed to the flow.
A distinctive feature of the Wright viscometer is the manner in which it monitors bob position. Specifically, the bob includes ferromagnetic material and is positioned with respect to two coils so that its movement changes the mutual inductance between them. The viscometer circuitry then senses the electromotive force that alternating current in one coil induces in the other coil, and the bob is inferred to have reached a predetermined point in its travel when the magnitude of the induced AC signal falls to a predetermined fraction of its peak value for the current bob stroke.
The circuit for achieving this result includes a peak detector, which holds as its output the peak induced voltage for the current bob stroke. A comparator compares a predetermined percentage, say 90%, of this peak voltage with the instantaneous induced voltage. So long as the coil output has not fallen to 90% of its peak value, therefore, the output of the comparator is a square wave. The viscometer circuitry infers that the bob has reached the predetermined position when the square wave stops.
A determination that the square wave has stopped is made by a retriggerable monostable multivibrator, which is triggered repeatedly by the leading edges of the square wave. The monostable multivibrator remains triggered--i.e., in its unstable state--until the square wave stops, at which point the triggering stops and the monostable multivibrator resumes its stable state after its characteristic period has been completed. Resumption of the stable state is the indication that the bob has reached the predetermined position.
The period of the monostable multivibrator must be at least as great as the period of the AC signal induced in the coil; otherwise, the monostable multivibrator would resume its stable state before the square wave stopped. In practice, moreover, it is desirable for the period of the monostable multivibrator to be several times the period of the AC signal, because such a period length increases the immunity of the system to noise that might cause individual square-wave pulses to be missed. As a consequence, the monostable multivibrator imposes a certain delay in the system: the monostable multivibrator output does not resume its stable state until a fixed period of time after the square wave has stopped and thus after the bob has reached the predetermined position. The delay of the monostable multivibrator, though necessary, is thus a source of some inaccuracy.
Of course, the inaccuracy as a fraction of the total measurement can be reduced by increasing the length of bob travel, and thus the stroke duration, or by decreasing the period of the monostable multivibrator. But the period of the monostable multivibrator can be reduced only to the period of the AC signal, which noise considerations often require to be relatively long, and it is not always practical to increase the length of the bob stroke. Therefore, the monostable multivibrator used to indicate the presence or absence of the square wave is a source of inaccuracy.
An object of one aspect of the present invention is to reduce the flow- and gravity-induced inaccuracies that result from variations in viscometer orientation.
An object of another aspect of the present invention to reduce the inaccuracy caused by the monostable multivibrator without increasing stroke lengths or the frequency of the coil signal.
Another object of the invention is to measure fluid viscosity effectively.